I normally don't use Compiz, but I just gave it a try. Unbelievable fglrx eating memory away like crazy in 8 MB chunks! And indeed a simple glxinfo releases the memory. I'm sure this is fglrx at fault and not X. top nor xrestop show a user process growing in memory usage. cat /proc/slabinfo seems to indicate a lot of kmalloc-8192 objects being allocated (and released when running glxinfo).